Copiers are often provided with an exposure platen cover which may be raised above the platen to accomodate copying of a single document from a three-dimensional object, such as a magazine or book or an oversized drawing. See, for example, U.S. Pat. Nos. 3,630,620 and 4,110,041. The exposure of an image on a photosensitive element such as a photoconductor is a function of the area under an Intensity (I) vs time(t) curve. When document images are exposed on a moving photoconductor, it is desirable to use flash lamps which produce a flash pulse that illuminates a document at the platen. Light reflected off the document exposes the photoconductor. Preferably, the duration of the flash pulse is made short to minimize image smear on the photoconductor. There is a practical limit as to how short the duration of the flash pulse can be made, which depends upon the construction of the flash lamps and driving power supply. A pulse with a short time period necessarily has high peak intensity to provide the appropriate exposure. An example of the exposure of an image on a photoconductor of a typical document by such a flash pulse is shown in FIG. 3. However, if the cover is raised, an operator's eye can be subjected to the high-peak intensity light. To protect an operator from potential eye damage, manufacturers have reduced the peak intensity of a pulse and increased the time period of the flash pulse. An example of the exposure of an image of a typical document on a photoconductor by this longer flash pulse is shown in FIG. 4. For a specific example, for the same document at the exposure platen, if the areas under the curves of FIGS. 3 and 4 are substantially the same, a photoconductor will be subject to the same exposure, whichever flash pulse is used. The period and intensity of this longer flash pulse can be determined empirically so that it causes exposure of an image on a photoconductor of a typical document which will provide copies, without appreciable image smear. There is a practical limit as to how long the duration of the flash pulse can be made, which depends upon the response characteristics of the photoconductor and the velocity which the photoconductor moves past the exposure platen during exposure.